[UPDATE April 18th 2013: this vulnerability has been fixed in both WP Super Cache and W3 Total Cache. You can find more information in this "post mortem" blogpost]
[UPDATE April 11th to reflect that WP Super Cache version 1.3 fixed this issue]
There was a pretty severe vulnerability in WordPress installations that had WP Super Cache (until version 1.2, 1.3 fixed this issue) or W3 Total Cache (up until version 0.9.2.8) plugins activated. This security bug would, under certain circumstances, allow attackers to inject and execute arbitrary PHP code in comments.
The vulnerability could have been handled in WordPress core or in WP Super Cache and W3 Total Cache separately (with my preference being a fix in comment sanitization in core). On April 11th WP Super Cache version 1.3 was released, fixing this issue and W3 Total Cache released a fix on April 18th. If you are on an older version of WP Super Cache or W3 Total Cache (do upgrade!), you might be interested in installing this little plugin that cleans out malicious … stuff from comments being posted.
As always; comments, bugs & improvements are welcome in the comment-field below or via the contact form.